In my experience (I've also had curl loss from henna) an amla pack will restore body and some curl. Repeated uses seem to be necessary for a longer term effect. Shikakai also seems to help. I do find amla a bit drying, so I've never used it straight, but only mixed with fenugreek/methi.
As to whether adding amla to subsequent hennas will prevent or lessen curl loss, I can't personally attest to that yet. I have been adding it to my root touch-up mixes for a while, but I've always had that 3a tendency of having straighter roots, and the hennaed/amlaed portions haven't grown down enough to judge yet.
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